1. Field of the Invention
The present invention relates to building structures and, more particularly, to structural techniques utilizing lightweight materials to perform a load bearing function.
2. Description of the Prior Art
Let us first consider the fundamental difference between systems of rigid versus non-rigid building construction systems. Historically, buildings evolved from compression structures built from materials such as stone and clay bricks that are completely rigid. For these materials to fail under load, requires extreme compressive force that will cause the structural members to be crushed or to fracture. The problem with such constructions is the excessive weight of the materials.
Advancements in materials and construction technology created new components and structural members designed to work in both compression and tension. These structural components and members are designed with sufficient stiffness to prevent a member from buckling under compression loads. Wood is among the early materials having both good compression and tensile strength. Modern materials technology has focused on the use of metal structural members that have equal or greater tensile strength than compressive strength. These structural members can be used to fabricate engineered structural components such as Open Web Joists that can be used to construct rigid, free span structures using a minimum weight of material. For a given weight a non-rigid metal member or cable acting purely in tension can carry a greater load over a given span than any of the above mentioned structural members.
Conventional metal structures are designed with strict tolerance in regard to the stiffness of members in bending, because deflection of a member under compressive load will cause the member to buckle resulting in immediate structural failure. Engineered components such as open web joists are very light in weight, and work equally well in compression or tension, but perform poorly in deflection. When joists are used in the construction of conventional flat roofs the combined live and dead loads must be specified with careful reference to building codes so that the joists will be sufficiently rigid to prevent ponding. State-of-the-art design of flat roofs strictly limits deflection and gives careful consideration to the drainage of water off the roof, especially in the case where the formation of ice or collection of debris may cause drainage problems. Neglect in these matters can ultimately result in ponding on the roof and lead to structural failure.
More recently flexible architectural fabrics have been used as the cladding or xe2x80x9cstressed skinxe2x80x9d of the building envelope. These thin cladding materials act purely in tension. Stressed skin methods of construction differ from traditional tent-like structures in that tension forces are introduced into the sheet material after it has been installed. Referred to as xe2x80x9cpost-tensionxe2x80x9d, it is this force that is used to stabilize the thin cladding or skin. In traditional tents a skin is fitted to the frame but not stressed and therefore it is free to deflect in it""s span between structural supports. Such traditional tents show noticeable movement and fluttering of the skin in the wind. Attempts to simply stretch the material tight are limited in their effectiveness, since a relatively weak force acting at a right angle to the skin will be able to significantly deflect the skin at the center span. Therefore, the modem approach is to use air pressure or structural tension members like cables to introduce a controlled amount of deflection and post-tension in the skin to create thereby a stressed skin structure.
High strength architectural fabrics have used air pressure and cable system post-tension methods in the construction of very large stressed skin roof systems such as Olympic Stadiums and airport terminal buildings. These large membrane roof systems have complex double curvature surface areas comprised of many individual membrane panels having irregular curved xe2x80x9csail shapesxe2x80x9d which are joined together to form the shape of roof. In the opposite extreme, weak film materials have been used to construct large area greenhouse structures where, for example, polyethylene film is used as a stressed skin over arched frames to cover agricultural crops. The film material is placed over the arches and cables or cords are placed over the top of the film between the frames to draw down and tension the film. Alternatively, a double layer of polyethylene film is attached over the arched frames and then air pressure is used to create an air pillow type of stressed skin.
It is evident that the gradual evolution of building design toward lightweight, flat roof, construction is driven by the efficiency of such systems. In general these systems cost less to build. The problem is that state-of-the-art building design for flat and low-profile roofs do not make use of the full potential of flexible sheet material and non-rigid structural members, that work most efficiently in tension, because of the ponding problem mentioned above.
Another problem of the prior art systems resides in the fact that many of the sheet materials show elastic elongation under load which can exaggerate the deflection that may be anticipated. This is an especially serious problem in the case of the solar sheet materials like transparent films and translucent membranes, which can be more elastic. Many excellent solar sheet materials with good tensile strength exhibit significant elasticity under load, which, especially for economical flat roof construction, necessitates the specific methods provided herein in order to avoid the serious problem of ponding.
Previous stressed roof panels, such as those described in my U.S. Pat. No. 4,452,230 issued on Jun. 5, 1984, are typically built with significant slope across the panels span, as this is known to be a requirement to prevent the collapse or inversion of the flexible sheet material or panels due to live loads. These previous types of structures, even though built with strong architectural fabric would, if built with insufficient slope, suffer from ponding due to high live loads caused by snow and rain. Deflection of the stressed skin or panel would cause the pooling and the collecting of water, snow or ice, generally referred to as xe2x80x9cpondingxe2x80x9d that then produces even greater loads in the area of inverted skin. Such ponding and inversion of the fabric stressed skin roof systems can then lead to roof collapse and structural failure. Therefore, these previous roof construction systems are not suited to flat, low profile roof systems, which are the most economical to build.
It is therefore an aim of the present invention to provide improved lightweight structures covered with flexible sheet materials.
It is also an aim of the present invention to provide an improved roof drainage system that prevents ponding on lightweight roofs covered with thin flexible sheet materials.
It is also an aim of the present invention to provide a new modular stressed-panel building envelope comprising a plurality of double-layer flexible panels.
The present invention also discloses improvements whereby the tensile strength of the building envelope materials are fully exploited while the structural members and/or the sheet materials are permitted to deflect in a pre-determined manner when under load.
Therefore, in accordance with the present invention, there is provided a modular stressed-panel building envelope, comprising a plurality of multi-layer flexible paneling modules adapted to be stretched between structural members, each of said multi-layer paneling modules having at least exterior and interior stressed layers defining a free space therebetween, said exterior and interior layers being operatively connected to work structurally in opposition to each other.
In accordance with a further general aspect of the present invention, there is provided a lightweight building construction system, comprising multiple similar stressed roof and wall paneling modules assembled together to form a modular building envelope, wherein each roof paneling module includes at least interior and exterior flexible layers stretched between spaced-apart structural elements, said interior and exterior flexible layers being joined together between said structural element to work in tandem once in a stressed state.
In accordance with a further general aspect of the present invention, there is provided a canopy system for a building structure, comprising panel means stretched in a V-shaped configuration between spaced-apart structural elements so as to define a trough therebetween, and gutter means hanging from said panel means at said trough for draining off water from said panel means, while allowing said gutter means to move jointly with said panel means.
In accordance with a further general aspect of the present invention, there is provided a flexible joist for a lightweight building structure, comprising a first elongated flexible member adapted to be supported in tension in an elevated position, a second elongated flexible member adapted to be supported in tension beneath said first elongated flexible member, and tensor means extending between said first and second elongated flexible members to induce opposing concave deflections in said first and second elongated flexible members, while preventing said first and second elongated flexible members from returning to respective relaxed positions thereof.
According to one application of the present invention, the construction methods disclosed in the present specification improve the performance and economy of a roof construction by means of an improved roof drainage system that prevents ponding on lightweight roofs covered with thin flexible sheet materials. These methods also optimize the structural performance of flexible sheet materials and metal structural components by taking advantage of their typical high tensile strength. In this specification, these improved methods are referred to generally as a xe2x80x9cstressed-panelxe2x80x9d system of construction. This term also refers to the specifications provided herein for the modularization of the roof and wall systems so that the sheet material is pre-manufactured as modular panels that increase the economy and speed of manufacturing and construction.
In one preferred embodiment of the present invention, transparent or translucent sheet materials are used in the construction of a type of lightweight structure referred to as a xe2x80x9cSolar Structurexe2x80x9d. The Solar Structure may be constructed with double or triple layers of transparent or translucent sheet material forming a building envelope wherein there are duct like cavity spaces formed between the sheet materials. Thin sheet materials having high optical clarity provide the best solar energy transmission for multiple layer construction. Often, good optical performance of the materials can conflict with obtaining the best structural strength and dimensional stability, and in such cases, the methods and devices disclosed herein have particular advantages. In general the optically transparent sheet materials or panels utilized in this construction may be referred to as films, while higher tensile strength, translucent, scrim or fabric reinforced sheet materials are referred to as membranes or architectural fabrics, respectively.